1. Field of the Invention
The present invention relates to the structure of an infrared detector with a unified absorption layer having low thermal mass and highly effective infrared absorbance and thus being capable of improving the characteristics of an uncooled infrared detector, and an uncooled infrared detector using this structure of an infrared detector with a unified absorption layer.
2. Description of the Related Art
The pyroelectric, bolometer, and thermopile being applied for uncooled infrared detector are a thermal detector, which detects infrared by transforming the change of physical characteristics in a material by thermal energy of incident infrared to electric signal. Therefore, for the purpose of fabricating a highly sensitive infrared detector, it is essential to make the physical characteristic change of the detecting material be bigger by increasing the absorbance of incident infrared.
Infrared absorption methods developed until now are divided into the method using infrared absorption materials such as a gold black and a platinum black, and the method absorbing infrared by resonating infrared using λ/4 optical depth.
While the method using a metal black can obtain high infrared absorbance over 90%, it is difficult to apply to FPA (focal plane array) for the real device and infrared imaging device by the patterning problem (even though the thickness of an absorption layer is from several μm to tens of μm, it is too thick and the metal blacks have a poor adhesion property, thereby the patterning problem can be occurred due to the characteristics of the material).
One of the methods using λ/4 optical depth absorbs infrared by forming a λ/4 resonating absorption structure of semi-transparent film/dielectric (generally polyimide)/reflecting film structure on the top side of an infrared detection part, as described in FIG. 2 and thus it has been generally applied to the infrared device fabricated by the bulk micromachining method. The methods using silicon, or germanium with high refractive index as dielectric material are disclosed in JP 13-116616 A, JP 13-153722 A, and JP 14-054995 A. These absorption layer structures have infrared absorbance over 90% similar to that of metal black absorption layer. However, in case of using polyimide, the thickness for polyimide is around 1.6 μm and thus it is thick. The specific heat capacity of polyimide is about 1100 J/K/kg and thus it is rather high and it causes thermal mass to be much increased. This thermal mass increment acts as the factor hindering the characteristic improvement of infrared device. Though the use of silicon or germanium with high refractive index can also reduce the thickness than the use of polyimide, the increment of thermal mass caused by the use of silicon or germanium is needed to be minimized.
The other method using λ/4 optical depth is to absorb infrared forming λ/4 air cavity resonating structure by making the floating height be 2.5 μm(λ/4 of 10 μm wavelength) using surface micromachining method as shown in FIG. 4. Since this method absorbs infrared using floating structure itself without special absorption layer, it can improve infrared absorbance without any thermal mass increment. However, it can cause the decrement of infrared absorbance because of the distortion of floating structure frequently occurred after the removal of a sacrificial layer in surface micromachining, and this method greatly depends on the reproducibility and uniformity of device fabrication process, and can produce uniformity problem of overall device due to the characteristic degradation of each individual pixel when it is applied to FPA.
The present invention is for resolving the above existing technical problems, and the objective of the present invention is to provide a novel absorber structure with high infrared absorbance over 90% as well as resolve the problem of high increment of thermal mass which is a problem in metal/dielectric/metal absorber structure used as an infrared absorption layer in the device fabricated with existing developed bulk micromachining method.
The other objective of the present invention is to provide an infrared device with a highly effective infrared absorber for a new structure of uncooled infrared detector which is capable of resolving the problem of the infrared absorbance decrement due to the distortion of floating structure occurred in λ/4 air cavity absorber structure fabricated by surface micromachining method.